Changes in REE fractionation induced by the halophyte plant Halimione portulacoides, from SW European salt marshes
Introduction
Lanthanides (LaLu), together with yttrium (Y) and scandium (Sc) constitute a group of 17 coherent elements, commonly named rare earth elements or REE (IUPAC, 2005). These elements are widely used in a large range of industrial applications (e.g. new materials and products, manufacture of catalysts, permanent magnets, glass and ceramics, green energy systems), in agriculture (e.g. fertilizers) and animal production (micronutrients for animal feed) and medicine (e.g. Gd-based contrast-agents used in magnetic resonance imaging – MRI) (Balaram, 2019). Because of REE widespread use, there has been an increase input into the aquatic environment, particularly in ecosystems closer to large urban areas, such as estuaries, whereas salt marshes have been recognised as natural sinks of REE (Balaram, 2019; Borrego et al., 2002; Brito et al., 2018b, Brito et al., 2019; Delgado et al., 2012; López-González et al., 2012; Merschel et al., 2015; Prego et al., 2009; Sklyarova et al., 2017; Turra, 2018). Estuarine marshes accumulate high contents of deposited fine-grained material rich in metals including REE, from different sources, such as urban runoffs, leaching from the agricultural fields or from the waste water outfalls (Brito et al., 2018b; López-González et al., 2012; Morgan et al., 2016; Wang et al., 2015). The physical and chemical characteristics of estuarine sediments (pH, salinity, redox potential, organic matter content and grain size) may influence the mobility and availability of metals absorption by halophyte plants. On the other hand, variations in metal speciation and availability are induced by changes in these factors due to plant activity (Reboreda and Caçador, 2007a; Windham et al., 2004).
Knowledge about the relationship between REE and plants is mainly focused on terrestrial species with a particular interest in human consumption (Tyler, 2004 and references therein). In a recent work on the effect of halophyte plants (Sarcocornia fruticosa and Spartina maritima) activity on REE, (Brito et al., 2018a) points to increased retention of lanthanides in the rhizosediment. Results also showed different REE fractionation patterns between roots and above-ground organs.
The sea purslane, Halimione portulacoides (L.) Aellen (Caryophyllales: Chenopodiaceae), is considered one of the most abundant salt marshes plants in Europe and one of the most productive species (Bouchard et al., 1998). This halophyte species has been widely reported in interrelations studies with contaminants in the plant-sediment system, being considered an effective accumulator for many trace metals and other pollutants (e.g. Andreotti et al., 2015; Caçador et al., 2009; Pedro et al., 2015; Reboreda and Caçador, 2007a, Reboreda and Caçador, 2007b), and recognised the important role halophyte plants play as phytoremediation agents in areas contaminated by trace metals (e.g. Kumari et al., 2019; Liang et al., 2017; Manousaki and Kalogerakis, 2011; Milić et al., 2012; Nikalje and Suprasanna, 2018; Weis and Weis, 2004). Several studies carried out with several halophyte species, including the H. portulacoides, in the Rosário salt marsh, stress the plants suitability for phytoremediation processes, either as phytostabilizers of sediments with excess of toxic ions or as trace metals phytoextractors. In a previous study, on spatial distribution and identification of REE sources in the Tagus estuary reported an active anthropogenic source associated with an old open-air phosphogypsum stack located near the Rosário salt marsh.
The main objectives of the present work were: a) to quantify the levels of REE in the different organs of H. portulacoides (roots, stems and leaves); b) to evaluate the translocation capacity of these metals between the various organs; c) to determine the H. portulacoides' role in the REE distribution and fractionation in the rhizosphere; and d) to evaluate the capacity and suitability of this halophyte species as a phytoremediation agent in sediments potentially contaminated by REE. Finally, and based on the results obtained, to contribute to the increase of scientific knowledge about the interactions of these plant species and the REE in estuarine salt marshes, environments of recognised high value, not only ecological but also socio-economic.
Section snippets
Study area
Fig. 1 shows the location of Rosário's salt marsh (38° 40.161′N, 9° 00.198′W), in the lower Tagus estuary (SW Europe) where this study was carried out. The estuary is characterized by a deep, straight and narrow inlet channel, and a wide, shallow inner bay with extensive marshes and salt flats as a result of the constant inlet of fine sediment (Brito et al., 2018a, Brito et al., 2018b and references therein). The marsh represents a typical estuarine salt marsh colonised by homogeneous groups of
Sediments characterization
The results of the sediment characterization exposed the different nature of the sedimentary material are presented in Table 1. The colonised sediment (sedHp) showed a particle size composition mostly composed of fine-grained material (87–96%), compared with the non-colonised sediment (sedNc), which revealed a mixture of sand (0.20–2.4%) and silt + clay (18–85%). The sedHp also contained the highest values (p < 0.001) of moisture content (% water content) and organic matter (% LOI). Values of
Discussion
The REE chemistry in estuarine sediments is mainly controlled by various physical and chemical processes that define dissolved and particulate metals partition (e.g. Elderfield et al., 1990; Sholkovitz, 1995; Sholkovitz and Elderfield, 1988), while the plants' capacity to accumulate these elements is related with concentrations in the sediment and their speciation, but also depends on the plant species (e.g. Tyler, 2004). Previous studies have shown the ability of salt marsh halophyte plants to
Conclusions
Although the accumulated REE values observed in this halophyte roots were still lower relative to the sediment contents, the new results obtained in this study with H. portulacoides indicate that the plants' ability to retain these metals (phytoextraction) may be ruled by the form on how REE are complexed rather the sediment metal contents. The enrichment of Y and MREE found in the non-colonised sediment, and in the H. portulacoides roots, with fractionation patterns like the
Acknowledgements
This work was financially supported by the Portuguese Science Foundation through the REEUSE Project (FCT/PTDC/QEQ-EPR/1249/2014).
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